1. Field of the Invention
The present invention relates to vehicle AC generators for equipping passenger vehicles and trucks, and in particular relates to a structure of a rectifier unit applied to the vehicle AC generator.
2. Description of the Related Art
FIG. 21 is a sectional view showing a structure of a conventional vehicle AC generator disclosed in Japanese Unexamined Patent Application Publication No. 8-182279, for example; FIG. 22 is a bottom plan view showing a rectifier unit for equipping the conventional vehicle AC generator; FIG. 23 is a schematic representation for showing an assembly procedure of a stator and the rectifier unit in the conventional vehicle AC generator; and FIG. 24 is a circuit diagram showing an example circuit of the conventional vehicle AC generator.
As shown in FIG. 21, the conventional vehicle AC generator is formed so that a Lundell-type rotor 7 is rotatably fitted in a case 3 formed of a front bracket 1 and a rear bracket 2, which are made of aluminum, via a shaft 6 while a stator 8 is secured to the internal wall of the case 3 so as to cover the external peripheral side of the rotor 7.
The shaft 6 is rotatably mounted on the front bracket 1 and the rear bracket 2 via a pair of bearings 14a and 14b, respectively. At one end of the shaft 6, a pulley 4 is secured so that a rotating torque of an engine can be transmitted to the shaft 6 via a belt (not shown). At the other end of the shaft 6, slip rings 9 for supplying an electric current to the rotor 7 are secured, so that a pair of brushes 10 are accommodated in a brush holder 11 disposed in the case 3 so as to touch the slip rings 9 slidably. A voltage regulator 17 for regulating the AC voltage generated in the stator 8 is adhered to a heat sink 18 fitted to the brush holder 11. A rectifier unit 12 electrically connected to the stator 8 for rectifying the AC current generated in the stator 8 into a DC current is equipped in the case 3.
The rotor 7 is formed of a rotor coil 13 for generating magnetic flux by passing through an electric current and a pair of Lundell-type pole cores 20 and 21 disposed so as to cover the rotor coil 13, magnetic poles being formed in the pair of pole cores 20 and 21 by the magnetic flux generated by the rotor coil 13. The pair of pole cores 20 and 21 are made of iron, each has a plurality of claw-shaped magnetic poles 22 and 23 projecting from an outer circumferential edge thereof at spaced even angular pitch in the circumferential direction, and the pole cores 20 and 21 are secured to the shaft 6 so as to mesh with the claw-shaped magnetic poles 22 and 23 by opposing them, respectively. Axial-flow fans. 5a and 5b are respectively secured as cooling means to both end faces of the pole cores 20 and 21 in the axial direction.
The stator 8 is formed of a stator core 15 and a stator coil 16 having a conductor wound around the sator core 15 and generating an AC current by changes in magnetic flux caused from the rotor 7 accompanied by the rotation of the rotor 7. Parts of the conductor wound around the stator core 15 extend to both ends of the stator core 15 in the axial direction so as to form a coil end 16f in the front side and a coil end 16r in the rear side, respectively. In addition, the stator coil 16 is an Y-three-phase winding formed by Y-connecting three coils.
The rectifier unit 12 is provided with diode 30a for the positive-electrode side, diode 30b for the negative-electrode side, a cooling plate 31 for the positive-electrode side for supporting the diode 30a, and a cooling plate 32 for the negative-electrode side for supporting the diode 30b. The cooling plates 31 and 32 have a plurality of straight heat-radiation fins 31a and 32a which protrude at right angle to the shaft 6 and extend in parallel with the shaft 6. The diodes 30a are aligned at predetermined intervals on the principal plane of the cooling plate 31, i.e., the plane opposite to the plane having the heat-radiation fins 31a formed thereon. The planar base electrode surfaces of the diodes 30a are connected to the principal plane of the cooling plate 31 by soldering. Similarly, the diodes 30b also are aligned at predetermined intervals on the principal plane of the cooling plate 32. The cooling plates 31 and 32 are combined such that the rear surfaces of the diodes 30a and 30b oppose each other diametrically. Pairs of leads 30al and 39bl of the diodes 30a and 30b extend in parallel to the shaft 6 together with connection terminals 33a of a circuit board 33 and are soldered to leads 16a through 16c and 16n of the stator coil 16 introduced by a guide 34a of a partition plate 34, respectively.
As shown by the circuit diagram in FIG. 24, the three-phase AC voltage generated from each output terminal of the stator coil 16 is rectified in the full-wave by three sets of diode bridges comprising the diode 30a and the diode 30b, and a ripple-current component is output by one set of diode bridges comprising the diode 30a and the diode 30b via an Y-connected neutral point of the stator coil 16 so that the output current is improved. Therefore, the rectifier unit 12 is provided with four diodes 30a for the positive-electrode side and four diodes 30b for the negative-electrode side.
The diode 30a for the positive-electrode side has a planar electrode in the cathode side of a unidirectionally conducting element and a lead 30al in the anode side of the unidirectionally conducting element, and are constructed to be of a substantially rectangular parallelepiped resin-molded type package. The diode 30b has planar electrode in the anodes side of the unidirectionally conducting element and a lead 30bl in the cathode side of the unidirectionally conducting element, and are constructed to be a substantially rectangular parallelepiped resin-molded type package.
In addition, in the case of the circuit diagram shown in FIG. 25, a rectifier unit 12A has only three sets of diode bridges comprising the diode 30a for the positive-electrode side and the diode 30b for the negative electrode side for performing full-wave-rectification of the three-phase AC voltage generated from each output terminal of the stator coil 16.
In the vehicle AC generator formed as above, a current is supplied from a battery (not shown) to the rotor coil 13 via the brushes 10 and the slip rings 9 so as to generate the magnetic flux. By the magnetic flux, the claw-shaped magnetic poles 22 of the pole core 20 are polarized with north-seeking (N) poles while the claw-shaped magnetic poles 23 of the other pole core 21 are polarized with south-seeking (S) poles. On the other hand, the rotating torque of the engine is transmitted to the shaft 6 via the belt and the pulley 4 so that the rotor 7 is rotated. Therefore, a rotating magnetic field is imparted to the stator coil 16 so as to generate an electromotive force in the stator coil 16. The AC electromotive force is rectified into a DC current by the rectifier unit 12, while the amount thereof is adjusted by the voltage regulator 17 so as to be charged in the battery.
In the vehicle AC generator, the rotor coil 13, the stator coil 16, the rectifier unit 12, and the voltage regulator 17 are generating heat at all times during the generation, wherein in the generator of a class operating at the rated 100 A output, they generate heat of 60 W, 500 W, 120 W. and 6 W, respectively, at a rotational speed showing high temperature. Therefore, in order to cool the heat produced by the generation, air intake openings 1a and 2a and air discharge openings 1b and 2b are provided in the front bracket 1 and the rear bracket 2, respectively. Specifically, as shown in FIG. 21, a plurality of air intake openings 1a and 2a are provided on the axial faces (end faces) of both the brackets 1 and 2 so as to oppose the rectifier unit 12 and the in voltage regulator 17, while a plurality of air discharge openings 1b and 2b are provided in the vicinities of the external peripheries of the axial-flow fans 5a and 5b on the radial faces (side faces) of both the brackets 1 and 2.
In the front side, as shown by the solid lines in FIG. 21, ambient air is sucked in the case 3 from the air intake openings 1a by the rotation of the axial-flow fan 5a in the front side, and then is curved in the centrifugal direction by the axial-flow fan 5a, and thereby cooling a coil end portion 16f in the front side of the stator coil 16 so as to be thereafter exhausted from the air discharge openings 1b. 
On the other hand, in the rear side, as shown by the dotted lines in FIG. 21, ambient air is sucked in the case 3 from the air intake openings 2a by the rotation of the axial-flow fan 5b in the rear side so as to cool the rectifier unit 12 and the voltage regulator 17, and then is curved in the centrifugal direction by the axial-flow fan 5b, and thereby cooling a coil end portion 16r in the rear side of the stator coil 16 so as to be thereafter exhausted from the air discharge openings 2b. 
Next, a conventional rectifier unit using a cooling plate formed to be substantially horseshoe-shaped will be described.
FIGS. 26 to 28 are a perspective view, a front view, and a rear elevational view, respectively, of another conventional rectifier unit disclosed in Japanese Unexamined Patent Application Publication No. 8-182279; FIG. 29 is a sectional view showing an essential part of the structure of the rectifier unit shown in FIG. 26; and FIGS. 30 and 31 are an essential part sectional view and an essential part front view, respectively, showing the flow of cooling air in a vehicle AC generator equipping with the rectifier unit shown in FIG. 26.
In the conventional rectifier unit 12B, cooling plates 35 and 36 for the positive- and negative-electrode sides are respectively formed in a horseshoe shape. The cooling plate 35 for the positive-electrode side has a plurality of heat-radiation fins 35a extending radially on the rear plane, i.e., the face opposite to the principal plane. Diodes 30a for the positive-electrode side are aligned at predetermined intervals in the circumferential direction on the principal plane of the cooling plate 35. The planar base electrode surface of the diode 30a is connected to the principal plane of the cooling plate 35 by soldering. Similarly, the diodes 30b also are aligned in the circumferential direction at predetermined intervals on the principal plane of the cooling plate 36. The cooling plates 35 and 36 are coaxially placed such that their principal planes are positioned on a plane orthogonal to the axis of the shaft 6. At this time, the diodes 30a and 30b oppose each other diametrically. Pairs of leads 30al and 30bl of the respective diodes 30a and 30b extend in parallel to the shaft 6 together with connection terminals 37a of a circuit board 37 and are soldered to leads 16a through 16c and 16n of the stator coil 16.
In the vehicle AC generator equipping with the rectifier unit 12B formed as above, as shown by the dotted lines in FIGS. 30 and 31, ambient air is sucked in the case 3 from the air intake openings 2a by the rotation of the axial-flow fan 5b in the rear side so as to impinge upon the cooling plate 35; flows along the heat-radiation fins 35a thereof toward the shaft 6; flows toward the rotor 7 via between the shaft 6 and the cooling plate 35; and then is curved in the centrifugal direction by the axial-flow fan 5b, and thereby cooling a coil end portion 16r in the rear side of the stator coil 16 so as to be thereafter exhausted from the air discharge openings 2b. 
Next, the conventional rectifier unit disclosed in Japanese Unexamined Patent Application Publication No. 7-231656 will be described. FIG. 32 is a front view of the conventional rectifier unit disclosed in Japanese Unexamined Patent Application Publication No. 7-231656, for example, and FIG. 33 is a sectional view showing a rectifier element forming the rectifier unit shown in FIG. 32.
In FIG. 33, the rectifier element 38 comprises a cylindrical metallic case 39 having a bottom, a unidirectionally conducting element 40b for the negative-electrode side with the anode side soldered to the internal bottom face of the case 39, a unidirectionally conducting element 40a for the positive-electrode side with the anode side soldered to the cathode side of the unidirectionally conducting element 40b via a planar metallic plate 41, a connection terminal 42 soldered to the cathode side of the unidirectionally conducting element 40a, and a sealing material 43 made from an insulating resin and packed into the case 39. The external peripheral surface of the case 39 is knurled. The planar plate 41 extends from the opening of the case 39 to form a lead 41a. Similarly, the connection terminal 42 extends from the opening of the case 39 to form a lead 42a. The rectifier element 38 forms a diode bridge comprising the unidirectionally conducting elements 40a and 40b for the positive- and negative-electrode sides.
In FIG. 32, a heat-radiation plate 44 has three concave portions formed at predetermined intervals. A terminal base 45 is constructed to be of an insulating-resin-molded type package comprising one of a first conductor portion 45a and three of second conductor portions 45b. The rectifier elements 38 formed as above are respectively press-fitted into the three concave portions formed on the principal plane of the heat-radiation plate 44; the terminal base 45 is placed on the heat-radiation plate 44; the lead 41a of each rectifier element 38 is soldered to the first conductor portion 45a; and the lead 42a of each rectifier element 38 is soldered to the second conductor portion 45b, and thereby forming a three-phase full-wave rectifier unit 12C.
Furthermore, the conventional rectifier unit disclosed in Japanese Unexamined Patent Application Publication No. 5-176539 will be described. FIG. 34 is a sectional view of the conventional rectifier unit disclosed in Japanese Unexamined Patent Application Publication No. 5-176539, for example.
In FIG. 34, a radiating fin 47 for the positive-electrode side is connected to cathodes of four diodes 46a for the positive-electrode side and to output terminals (not shown). An anodes of each diode 46a for the positive-electrode side is individually connected to four cathodes of the diodes 46b via an AC heat-radiation fin 48, which is individually connected to the leads of the stator coil. An anode of each diode 46b for the negative-electrode side is connected to a heat-radiation fin 49 for the negative-electrode side, which is grounded via a frame 50. A rectifier unit 12D formed as above forms a four-phase full-wave-rectifying bridge.
The heat-radiation fin 49 for the negative-electrode side is formed by bending an aluminum plate to have a channel shape and has both ends fixed to the frame 50 with screws 51. The central portion of the heat-radiation fin 49 extends in parallel with the surface of the frame 50 leaving a predetermined space therebetween. An indented hole 52 and a ventilation hole 53 are formed in the central portion of the heat-radiation fin 49. Into the indented hole 52, a knurled portion 56 forming the anode of the diode 46b for the negative-electrode side is press-fitted and is fixed, while the body portion of the diode. 46b is disposed in the side of the frame 50 of the heat-radiation fin 49.
A knurled portion 56 forming the cathode of the diode 46b extends downwardly, when viewed in FIG. 34, and is press-fitted into the upper half of an indented hole 54 of the rectangular and planar AC heat-radiation fin 48 made of an aluminum plate. Into the lower half of the indented hole 54 of the AC heat-radiation fin 48, a knurled portion 56 forming the anode of the diode 46a for the positive-electrode side is press-fitted, and the knurled portion 56 forming the cathode of the diode 46b directly abuts the knurled portion 56 forming the anode of the diode 46a. 
A knurled portion 56 forming the cathode of the diode 46a is press-fitted into an indented hole 55 of the rectangular and planar heat-radiation fin 47 made of an aluminum plate, which is pushed to the rear end wall of the frame 50 via an insulating sheet 57.
FIG. 35 is a sectional view of another conventional rectifier unit disclosed in Japanese Unexamined Patent Application Publication No. 5176539.
In the rectifier unit 12E shown in FIG. 35, the heat-radiation fin 47 for the positive-electrode side is used as a bending member at the outermost section instead of the heat-radiation fin 49 for the negative-electrode side, and the frame 50 is used as both the frame and the heat-radiation fin 49. The heat-radiation fin 47 is fixed to the frame 50 with resin screws 51a via the insulating sheet 57.
The simplification of the structure of the rectifier unit 12E is made by such a manner.
In the conventional rectifier unit 12 shown in FIG. 22, since the diodes 30a and 30b for the positive- and negative-electrode sides are separately formed, a large part of the heat generated in the diodes 30a and 30b is thermally conducted to the cooling plates 31 and 32 for the positive- and negative-electrode sides, respectively, so as to be dissipated therefrom. A part of the heat generated in the diode 30a (30b) is thermally conducted from the lead 30al (30bl) to the cooling plate 32 (31) via the connection terminal 33a and the lead 30bl (30al) so as to be dissipated from the cooling plate 32 (31). However, since the thermal resistance between the lead 30al, the connection terminals 33a, and the lead 30bl is large, the structure is formed to be difficult to perform heat-exchange between the diodes 30a and 30b with each other. That is, the heat generated in the diodes 30a and 30b is dissipated independently from the cooling plates 31 and 32, respectively, so that when the heating value of the diode 30a is unbalanced with that of the diode 30b, there has been a problem that the diode having a larger heating value cannot be effectively cooled. That is, there has been a problem that increasing in temperature of the diode having a larger heating value cannot be restrained. In addition, the conventional rectifier unit 12B shown in FIG. 26 also involves this problem.
Also, in the conventional rectifier unit 12B, the cooling plates 35 and 36 for the positive- and negative-electrode sides formed in a horseshoe shape are coaxially placed such that their principal planes are positioned on a plane orthogonal to the axis of the shaft 6, so that the passage for cooling air between the cooling plate 35 and the shaft 6 is narrow, and thereby increasing the ventilation resistance so as to reduce the amount of cooling air. Consequently, there has been a problem that increasing in temperatures of the rectifier unit 12B and the stator coil 16 cannot be restrained.
Furthermore, in the conventional rectifier units 12, 12A, and 12B, when forming n sets of three-phase full-wave rectifier units, 12n or 16n diodes 30a and 30b for the positive- and negative-electrode sides are needed, so that the man-hour of joining operations between the diode and the cooling plate and between the diode and the lead of the stator coil is increased, and thereby raising a problem that the productivity is reduced and space saving is not achieved as well so as to increase the size of the vehicle AC generator.
Also, in the conventional rectifier unit 12C shown in FIG. 32, since the unidirectionally conducting elements 40a and 40b for the positive- and negative-electrode sides are deposited in one piece, when forming n sets of three-phase full-wave rectifier units, it is advantageous in view of improvement in productivity and miniaturizing of the vehicle AC generator because the number of the rectifier elements 38 is reduced. However, the joining area between the connection terminal 42 and the unidirectionally conducting element 40a for the positive-electrode side is small relative to the base area in the cathode side of the unidirectionally conducting element 40a, the thermal resistance between the unidirectionally conducting element 40a and the connection terminal 42 becomes large. Also, since the conductor portion 45a, which is connected to the lead 42a of the connection terminal 42, is molded with an insulating resin, thermal dissipation of the conductor portion 45a is small. Therefore, the heat generated in the unidirectionally conducting element 40a for the positive-electrode side is thermally conducted to the heat-radiation plate 44 via the unidirectionally conducting element 40b for the negative-electrode side and the case 39 to be dissipated from the heat-radiation plate 44, and thereby raising a problem that temperature increase in the unidirectionally conducting element cannot be effectively restrained.
In the conventional rectifier unit 12D shown in FIG. 34, the knurled portion 56 forming the cathode of the diode 46a for the positive-electrode side is press-fitted into the indented hole 55 of the heat-radiation fin 47 for the positive-electrode side, which is pushed to the frame 50 via the insulating sheet 57. Therefore, the heat generated in the diode 46a for the positive-electrode side is thermally conducted to the heat-radiation fin 47 for the positive-electrode side; however the heat in the heat-radiation fin 47 is difficult to be thermally conducted to the frame 50, and thereby raising a problem that temperature increase in the diode 46a for the positive-electrode side cannot be effectively restrained. When the thickness of the insulating sheet 57 is reduced, electric corrosion is produced between the heat-radiation fin 47 and the frame 50 so as to cause failed generation.
In the conventional rectifier unit 12E shown in FIG. 35, the knurled portion 56 forming the cathode of the diode 46a is press-fitted into the indented hole 55 of the heat-radiation fin 47 for the positive-electrode side so as to be fixed to the frame 50 with the resin screws 51a via the insulating sheet 57. Therefore, the heat generated in the diode 46a for the positive-electrode side is thermally conducted to the heat-radiation fin 47 for the positive-electrode side; however the heat in the heat-radiation fin 47 is difficult to be thermally conducted to the frame 50, and thereby raising a problem that temperature increase in the diode 46a for the positive-electrode side cannot be effectively restrained. There also has been a problem that the screw 51a is subject to a large sheared stress to be damaged because large vehicle vibration and the thermal cycle .are applied to the resin screw 51a. 
In the conventional rectifier units 12D and 12E, since the process of press-fitting the knurled portions 56 into the indented holes 52, 54, and 55 is repeated during the assembling the diodes 46a and 46b for the positive- and negative-electrode sides, so that a compression load is applied to the diodes 46a and 46b, and thereby raising a problem of the damaged diodes 46a and 46b. 
Furthermore, in the conventional rectifier units 12D and 12E, since the screws 51 and 51a for fixing are needed for every pair of the diodes 46a and 46b for the positive- and negative-electrode sides, the number of parts is increased and the space for fixing is needed as well. Therefore, when forming n sets of three-phase full-wave rectifier units, there has been a problem that productivity is reduced and the size of the vehicle AC generator is increased as well.